The present invention relates to an electric motor control device and a hybrid drive unit.
Japanese Patent Application Publication No. JP-A-7-303302 describes an electric motor drive employing a two-phase modulation system in which one phase voltage of three phase voltages applied to an electric motor is fixed at a high level or a level, and the remaining two phases are PWM-controlled. The two-phase modulation generates distortion in a waveform of the voltage applied to the electric motor, causing noise and degradation in efficiency. In order to improve these problems, Japanese Patent Application Publication No. JP-A-7-303302 presents a control system for reducing the distortion in the waveform. In addition, because the modulation becomes equivalent to a three-phase modulation if a shift amount of a triangular wave becomes 0 in a stage to generate a PWM waveform in the two-phase modulation, Japanese Patent Application Publication No. JP-A-7-303302 presents a switching control between two-phase and three-phase in which the modulation system is switched to the three-phase modulation system by specifying the shift amount to 0.
In a range in which a rotational speed of the motor is comparatively low, uncomfortable high-frequency noise is large if a carrier frequency of PWM pulses is low, whereas a switching loss in an inverter increases if the carrier frequency is increased. Therefore, in order to improve these problems, Japanese Patent No. JP-B2-3837986 presents a motor control device that increases the carrier frequency in the range in which the rotational speed of the motor is comparatively low and reduces the carrier frequency in a range in which the rotational speed of the motor is high. In addition, in the range in which the rotational speed of the motor is high, the motor is energized by rectangular waves, and in the range in which the rotational speed of the motor is low, the motor is energized by sine waves, by partial sine waves, or in an overlapping manner.
Japanese Patent Application Publication No. JP-A-2004-289985 describes inverter control that uses the three-phase modulation and a carrier frequency of a high value (7 kHz) in the case of a high target torque at a high-speed rotation, and uses the two-phase modulation and the carrier frequency of a low value (5 kHz) in other cases. Japanese Patent Application Publication No. JP-A-2004-289985 also describes a selection between the two-phase modulation and the three-phase modulation, as well as switching of the carrier frequency, and shows voltage waveforms applied to the electric motor in the two-phase modulation with a carrier frequency of 5 kHz, in the three-phase modulation with a carrier frequency of 5 kHz, in the three-phase modulation with a carrier frequency of 7 kHz, and in the two-phase modulation with a carrier frequency of 7 kHz, respectively, in FIGS. 4 to 7. Japanese Patent Application Publication No. JP-A-2004-289985 also shows a hysteresis in a frequency switching between the two-phase modulation with 5 kHz and the three-phase modulation with 7 kHz, in FIG. 8.
It is a problem that side band noise is included in an audio frequency band while a vehicle is running at a speed in the vicinity of 20 mph (miles per hour) to 50 mph (with a rotation of 2000 rpm to 8000 rpm, and a torque of −100 Nm to +100 Nm in a motor speed range). As a method to reduce the side band noise of the vehicle by inverter control, a method of increasing the carrier frequency is known. For example, according to Japanese Patent No. JP-B2-3837986, the noise is reduced by increasing the carrier frequency in the low-speed range. In general, the modulation system (voltage control mode) is not changed when the carrier frequency is switched to suppress the side band noise. Although, according to Japanese Patent Application Publication No. JP-A-2004-289985, the switching is performed between the two-phase modulation with 5 kHz and the three-phase modulation with 7 kHz (FIGS. 8 and 9, paragraph 0059), it is described that the switching is used to suppress a protective operation (shutdown) caused by a large drop in an inverter input voltage.